Cylinder 2 and 4 arent firing.

[Dibs]

1981 cb750c

Electronic ignition (old)
Mikuni carbs (new)
New coils, plugs, wires, ignitors

Could i have a bad regulator or bad electronic ignition part?

 

[dirtdigger]

Do compression test make sure you dont have low compression. Did you check if it had spark? Did you try to spray some carb clean in the carbs with running see if it helps?

 

[Dibs]

Do compression test make sure you dont have low compression. Did you check if it had spark? Did you try to spray some carb clean in the carbs with running see if it helps?

Did a compression test a while ago, it seemed to be good.

i checked valve clearances and all were in tolerance but i may check again.

I put on new coils, and have them routed correctly.

I have brand new tuned carbs on this thing, mikuni vm34. 2 of them instead of 4.

The 2 each have a split manifold, 1 carb splits into 1 and 2 and the other into 3 and 4.

Also, the spark plug wires are 1,4 and 2,3....
So why is 2 and 4 not running?

The adjacent cylinders are running with the same carbs and the coil is firing for both as cyl 2 is on a diff coil than 4
But 1 and 3 are firing off fine.

They are on seperate carbs and seperate coils.

There is a electronic ignition module instead of points but i dont know how it works.
I did adjust the timing were the marks are between the notch when the light flashes. It seemed to start easier and all that.

Could the electronic ignition be faulty?
Really banging my head against a wall here.

 

[Rampton]

First thing Todo is to check if you have a spark on all plugs remove #1 plug and insert into plug cap hold with a pair of plyers to cylinder head, turn bike over ensure kill switch is in run position and see if you have a bright blue spark, repeat for numbers 2,34 spark plug. If you have good spark on all, then problem is elsewhere fuel or compression. Ensure battery is fully charged before spark check. If you have no spark at the plugs you need to work backwards testing resistance (ohms) new sparkplugs check caps, HT leads, coil (1 coil for 1&4 1 coil for 2&3) replace if any found faulty. If all is well you need to test the pick up coils (electronic points) red plug left side of bike near the regulator test for continuity and resistance as per manual. Some times the pick up coils wires can short out due to old age. You can check the pick up coils visually by removing the round cover left side of bike, they have a air gap of about 0.008", if all is well then the next step is the ignitors two small boxes usually with red leads but these usually last a long time. Lots of u tube vids on how to check from battery to plug for resistance and continuity. I had a problem similar to yours and it turned out to be two problems faulty spark plugs and one side of pick coils shorted. Hope this helps and good luck from rainy England

 

[Dibs]

First thing Todo is to check if you have a spark on all plugs remove #1 plug and insert into plug cap hold with a pair of plyers to cylinder head, turn bike over ensure kill switch is in run position and see if you have a bright blue spark, repeat for numbers 2,34 spark plug. If you have good spark on all, then problem is elsewhere fuel or compression. Ensure battery is fully charged before spark check. If you have no spark at the plugs you need to work backwards testing resistance (ohms) new sparkplugs check caps, HT leads, coil (1 coil for 1&4 1 coil for 2&3) replace if any found faulty. If all is well you need to test the pick up coils (electronic points) red plug left side of bike near the regulator test for continuity and resistance as per manual. Some times the pick up coils wires can short out due to old age. You can check the pick up coils visually by removing the round cover left side of bike, they have a air gap of about 0.008", if all is well then the next step is the ignitors two small boxes usually with red leads but these usually last a long time. Lots of u tube vids on how to check from battery to plug for resistance and continuity. I had a problem similar to yours and it turned out to be two problems faulty spark plugs and one side of pick coils shorted. Hope this helps and good luck from rainy England

Beautiful reply, thank you. The pick up coils are the magnetic black boxes where the points would be if it had them, correct? I will look up the resistance check. I am less skilled with electical than i would like.

 

[Rampton]

Yep that's correct (electronic points) the air gap is the gap between the pointer and rotor less is best. Hope you get it sorted, lovely bike to ride

 

[Dibs]

Yep that's correct (electronic points) the air gap is the gap between the pointer and rotor less is best. Hope you get it sorted, lovely bike to ride

The pointer is on the rotor right? Where is the gap that needs to be checked to .008?

I already replaced the ignitors also.

I feel like i am very close to getting it sorted.

 

[Rampton]

On each of the black pick up coils there is a tiny piece of each unit that points towards the rotating shaft if you look closely you will see the pointer it's sticks out of the unit about 2mm or so as the shaft rotates it picks up the signal from pick up coils. The gap is from the tip of the pointer to the rotating shaft. Hard to explain because it is very small but if you look carefully you can see it. If required you loosen the two screws on the pick up coils and move the box to close or open the gap once set re-tighten the screws. I hope this helps you. Have a good day

 

[Dibs]

On each of the black pick up coils there is a tiny piece of each unit that points towards the rotating shaft if you look closely you will see the pointer it's sticks out of the unit about 2mm or so as the shaft rotates it picks up the signal from pick up coils. The gap is from the tip of the pointer to the rotating shaft. Hard to explain because it is very small but if you look carefully you can see it. If required you loosen the two screws on the pick up coils and move the box to close or open the gap once set re-tighten the screws. I hope this helps you. Have a good day

Thank you so much. I completely understand what your saying, thank you again.

 

[Rampton]

 

[Rampton]

You can see the pointer on this old set upper right of right pick up coil. Also brown wires bared shorting out as they go into the box was my problem. Hope this helps pal. Good luck

 

[Dibs]

You can see the pointer on this old set upper right of right pick up coil. Also brown wires bared shorting out as they go into the box was my problem. Hope this helps pal. Good luck